In a closed-loop emission control apparatus for internal combustion engines, the concentration of an exhaust composition is detected by a sensor as a feed-back control signal used to control the air-fuel ratio of the mixture supplied to the engine. The exhaust composition sensor such as zirconium dioxide type oxygen sensor normally generates an output which drops sharply in amplitude at stoichiometry as the detected oxygen concentration increases. More particularly, the output of the sensor is high for rich mixtures and low for lean mixtures. The output from the sensor is compared with a reference level that corresponds to a desired air-fuel ratio in the vicinity of stoichiometry to generate an error compensation signal indicative of the deviation of the mixture from the desired air-fuel ratio. However, the performance of the sensor tends to vary with time such that its sharp transient characteristic is lost and the knee portion of the curve occurs at a lower voltage level than in the earlier stage of use with the result that the reference level no longer coincides with the stoichiometric point of the mixture; specifically, it coincides with a point slightly richer than the desired stoichiometric value, and that for mixtures richer than stoichiometry the sensor delivers a lower voltage than that it is designed to deliver.